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<H1>Custom error responses</H1>

<DL>
<DT>Purpose
<DD>Additional functionality. Allows web-masters to configure the response of
Apache to some error or problem.<BR>
<P>Customizable responses can be defined to be activated in the event of a
server detected error or problem.<BR>
e.g. if a script crashes and produces a "500 Server Error" response, then
this response can be replaced with either some friendlier text or by a
redirection to another URL (local or external).
<DT>Old behavior
<DD>NCSA httpd 1.3 would return some boring old error/problem message which
would often be meaningless to the user, and would provide no means of logging
the symptoms which caused it.<BR><BR>
<DT>New behavior
<DD>The server can be asked to;
<OL>
<LI>Display some other text, instead of the NCSA hard coded messages, or
<LI>redirect to a local URL, or
<LI>redirect to an external URL.
</OL>
<P>Redirecting to another URL can be useful, but only if some information
can be passed which can then be used to explain and/or log the error/problem
more clearly.<BR>To achieve this, Apache will define new CGI-like environment
variables, e.g.
<blockquote><code>
REDIRECT_HTTP_ACCEPT=*/*, image/gif, image/x-xbitmap, image/jpeg <br>
REDIRECT_HTTP_USER_AGENT=Mozilla/1.1b2 (X11; I; HP-UX A.09.05 9000/712) <br>
REDIRECT_PATH=.:/bin:/usr/local/bin:/etc <br>
REDIRECT_QUERY_STRING= <br>
REDIRECT_REMOTE_ADDR=121.345.78.123 <br>
REDIRECT_REMOTE_HOST=ooh.ahhh.com <br>
REDIRECT_SERVER_NAME=crash.bang.edu <br>
REDIRECT_SERVER_PORT=80 <br>
REDIRECT_SERVER_SOFTWARE=Apache/0.8.15 <br>
REDIRECT_URL=/cgi-bin/buggy.pl <br>
</code></blockquote>

note the <code>REDIRECT_</code> prefix. <p>

At least <code>REDIRECT_URL</code> and <code>REDIRECT_QUERY_STRING</code> will
be passed to the new URL (assuming it's a cgi-script or a cgi-include). The
other variables will exist only if they existed prior to the error/problem.<p>

<DT>Configuration
<DD><em>file: </em>server configuration<BR>
<P>Here are some examples...
<blockquote><code>
ErrorDocument 500 /cgi-bin/crash-recover <br>
ErrorDocument 500 "Sorry, our script crashed because %s. Oh dear<br>
ErrorDocument 500 http://xxx/ <br>
ErrorDocument 404 /Lame_excuses/not_found.html  <br>
ErrorDocument 401 /Subscription/how_to_subscribe.html
</code></blockquote>
The syntax is, <p>
<code><A HREF="mod/core.html#errordocument">ErrorDocument</A></code>
&lt;3-digit-code&gt; action <p>

where the action can be,
<OL>
<LI>Text to be displayed.<BR>Prefix the text with a quote (&quot;). Whatever
follows the quote is displayed. If the error/problem produced any additional
information, it can be specified using <code>%s</code>.
<em>Note: the (&quot;) prefix isn't displayed.</em>
<LI>An external URL to redirect to.
<LI>A local URL to redirect to.
</OL>
<P><code>ErrorDocument</code> definitions are sensitive to a
<code>SIGHUP</code>, so you can change any of the definitions or add new ones
prior to sending a <code>SIGHUP</code> (kill -1) signal.
</DL>
<P><HR><P>

<h2>Custom error responses and redirects</H2>
<DL>
<DT>Purpose
<DD>Apache's behavior to redirected URLs has been modified so that additional
environment variables are available to a script/server-include.<p>

<DT>Old behavior
<DD>Standard CGI vars were made available to a script which has been
redirected to. No indication of where the redirection came from was provided.
<p>
<DT>New behavior
<DD>A new batch of environment variables will be initialized for use by a
script which has been redirected to.<BR>
Each new variable will have the prefix <code>REDIRECT_</code>.<BR>
REDIRECT_ environment variables are created from the CGI environment
variables which existed prior to the redirect, they are renamed with a
REDIRECT_ prefix, i.e. HTTP_USER_AGENT -&gt; REDIRECT_HTTP_USER_AGENT.<BR>
In addition to these new variables, Apache will define
<code>REDIRECT_URL</code> and <code>REDIRECT_STATUS</code> to help the script
trace its origin.<BR>
Logging: both the original URL and the URL being redirected to, will
now be logged correctly in the access log.<p>
</DL>

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